I've got to say, and I was just thinking about this earlier, that Satriani's soloing is melodic - by that I almost mean linear - compared to Vai. Satriani works the scales like they're just that, scales, in my opinion. I also think Vai composes some better tunes. I've only heard Satriani's album 'lost in space' though.

Vai is a bit more creative I guess. I hadn't heard of either before I came to this site.